Newton's Universe

In the ancient European tradition, stars were thought to sit on a huge rotating sphere that enclosed the Universe. People imagined the Universe as large, but conceivably large.

After the work of Kepler, the idea of celestial spheres started to lose its appeal. Eventually stars were understood to be individual suns, each one at a different distance away. Some were close and some were far. There was no longer any outer limit to the Universe.

Not only did ideas change about the size of the Universe in this time, they also changed about its nature.

Before Galileo, planets were thought to be made from different substance to the objects on Earth. This idea of Aristotle lasted for over 1000 years. However between the time of Galileo and that of Newton, people came to think of the stars and planets to be made of the same substance as Earth.

Newton's equation of gravity was Universal. It assumed that matter was the same everywhere in the Universe. The moons of Jupiter responded to gravity in exactly the same way that a person standing on Earth did, so they must be made from the same kind of matter.